2478
Solute-Solvent Interactions. IV. Infrared Studies of Solvent Effects on Rotational Isomers of 1,2-Dichloroethane and 1,I ,2,2-Tetrachloroethane Naobumi Oil and J . F . Coetzee’ Contribution f r o m the Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15213. Received November 13, 1968
Abstract: The influence of solvents on the C-CI stretching absorption bands of the trans and gauche isomers of 1,2-dichloroethane and 1,1,2,2-tetrachIoroethane was examined in terms of both relative frequency shifts and relative band intensities. The results indicate that the interaction of 1,2-dichloroethanewith the solvents studied is mainly dielectric in nature. However, with 1,1,2,2-tetrachIoroethanehydrogen bonding also becomes significant in proton-acceptor solvents, and it occurs with both isomers. A reasonably linear relationship exists between the relative intensities of the C-CI stretching bands of the rotational isomers of each of these two solutes and the relative frequency shifts of the C-CI stretching bands of n-propyl chloride or isobutyl chloride. This fact and also other evidence suggests that the main interaction responsible for the variation in relative stability of the two isomers in different solvents is dielectric in nature, and that the contribution of hydrogen bonding is less significant. It is speculated that the anomalous stabilization of the gauche isomer observed in aromatic solvents also is caused by an electrostatic rather than a hydrogen-bonding interaction.
P
revious studies on rotational isomerism3 showed that solvents affect the energy difference of rotational isomers. It was c ~ n c l u d e d ~that - ~ this solvent influence is largely dielectric in nature, in that the energy decrease caused by an increase in the bulk dielectric constant of the solvent is more marked for the more polar isomer. One of the most typical examples studied is 1,2-dichloroethane (DCE). Thus Hallam and Ray’ showed that the solvent shifts of the C-C1 stretching frequencies of DCE are greater for the more polar (gauche) isomer. Earlier, had found that the intensity ratio of the Mizushima, et Raman line at 754 cm-’ (assigned to the trans isomer) to that at 654 cm-’ (gauche) decreased from 5 to 1.2 as the dielectric constant of the solvent was increased from 2 (hexane) to 33 (methanol). However, in benzene as solvent an anomalous result was obtained, in that the above Raman intensity ratio indicated a relatively larger number of gauche molecules than would be expected from the low dielectric constant of benzene. Dipole moment measurement^^'^'^ showed the same anomaly, and Kubo8 inferred the formation of a 1 :1 complex between DCE and benzene. However, Kuratanig reported that the infrared absorption frequencies of DCE are similar in benzene and in carbon tetrachloride, carbon disulfide, and acetonitrile, and therefore discounted such complex formation. On the other hand, Neckel and Volkl’ obtained infrared shifts for DCE on adding benzene, and believed that the gauche form is preferentially stabilized by hydrogen bonding with ben(1) On leave of absence from the Central Research Laboratory, Sumitomo Chemical Co., Ltd., Takatsuki City, Osaka, Japan. (2) Please address all correspondence to this author. (3) S. Mizushima, “Structure of Molecules and Internal Rotation,” Academic Press, New York, N. Y., 1954. (4) S. Mizushima, Y. Morino, and K. Higasi, Sci. Papers Inst. Phys. Chem. Res. (Tokyo), 25, 159 (1934). ( 5 ) I. Watanabe, S. Mizushima, and Y . Masiko, ibid., 40, 425 (1943). (6) Y. Morino, S. Mizushima, K. Kuratani, and M. Katayama, J . Chem. Phys., 18, 754 (1950). (7) H. E. Hallam and T. C. Ray, J . Chem. Soc., 318 (1964). (8) M. Kubo, Bull. Inst. Phys. Chem. Res. (Tokyo), 13, 1221 (1934). (9) K. Kuratani, Rept. Inst. Sci. Technol., Univ. Tokvo, 6 , 221 (1952). (10) A. Neckel and H. Volk, 2.Elekfrochem., 62, 1104 (1958).
Journal of the American Chemical Society 91:lO May 7, 1969
zene. Nevertheless, no evidence was presented to rule out the possibility that the trans isomer may be involved in similar hydrogen bonding. Wada and Morinol also felt that a simple electrostatic model cannot account for the stabilization energy of DCE in benzene, and that some short-range interaction must be invoked. Recently, Chitoku and Higasi” concluded from dielectric relaxation measurements of DCE in several nonpolar solvents that the gauche form is preferentially stabilized in aromatic solvents by a weak interaction believed to involve hydrogen bonding. The conclusions to be drawn from the above studies are that most, but not all, of the evidence points to a specific interaction of DCE with benzene, and that this interaction probably is more pronounced for the gauche isomer and tentatively has been attributed to hydrogen bonding, but that its exact nature is not clear. In a previous paper13 we have shown that while the solvent shifts of the C-Cl stretching frequencies of chlorinated hydrocarbons generally do depend upon the bulk dielectric constant of the solvent, an important contributing effect arises when the hydrogen atoms attached to the carbon of the C-CI linkage have the ability to form hydrogen bonds with proton-acceptor solvents. In view of the unanswered questions summarized above, we have extended our work to a study of solvent effects on rotational isomerism. We have made further measurements with DCE, because intensity ratios for its rotational isomers have been reported for three solvents only.’ In addition, we have carried out a similar study with 1,1,2,2tetrachloroethane (TCE), which also exists in both trans and gauche forms,14 and which should be considerably more effective than DCE as a hydrogen-bond donor. Furthermore, in benzene as solvent its dipole moment is anomalously high.15 As in our previous paper, we refer (11) A. Wada and Y. Morino, J. Chem. Phys., 22, 1276 (1954). (12) K. Chitoku and K. Higasi, Bull. Chem. SOC.Japan, 40, 773 (1967). (13) N . Oi and J. F. Coetzee, J . Am. Chem. SOC.,91, 2473 (1969). (14) K. Naito, I. Nakagawa, K. Kuratani, I. Ichishima, and S. Mizushima, J . Chem. Phys., 23, 1907 (1955). (15) 0. Kiyohara and K. Higasi, Abstracts of Papers, Symposium on Molecular Structure, Sapporo, Japan, Oct 7, 1967.
2479 Table I. Solvent Effects on the C-CI Stretching Absorption Bands of 1,2-Dichloroethane
T
v17. C
Solvent
1 2 3 4 5 6 6' 7 8 9 10 11 12 13 14 15 16
Vapor n-Hexane Cyclohexane Pentene 1,4-Dioxane Carbon tetrachloride Benzene Benzene-d p-Xylene Mesitylene Tetrachloroethylene Triethylamine Carbon disulfide Diethyl ether Ethyl acetate Acetone Acetonitrile Nitromethane
v, cm-'
E"
G1
- CI
V'I~.C-CI
v, cm-'
103Av/v
727= 1.9 2.0 2.1 2.2 2.2 2.3 2.3 2.3 2.3 2.3 2.4 2.6 4.3 6.0 20.7 36.0 36.7
716 715 707
16.5 27.5
710.5 710.5 711 714
22.7 22.7 22.0 17.9
714 711 709 708 708 707.5
17.9 22.0 24.8 26.1 26.1 26.8
15.1
G2
(gauche)b 103Av/v
694" 686.5 686 685.5 673.5 685
10.8 11.5 12.2 29.5 13.0
679 680
21.6 20.2
685 683.5 683.5 680 673.5 672 672
13.0 15.1 15.1 20.2 29.5 31.7 31.7
v'5.c-
cI (gauche)b
v, cm-'
103Av/v
66gC 662.5 662 661 653.5 660.5
9.7 10.5 12.0 23.2 12.7
665.5 657 657.5 660.5 659 659 657.5 654.5 653 653
20.2 17.9 17.2 12.7 15.0 15.0 17.2 21.7 23.9 23.9
Ratios of peak intensities AT/AG, AT/AG,
8.2 8.7 4.0
13.5 12.3 4.3
4.9 5.8 7.9
4.8 5.4 5.9 9.9
7.2 5.7 3.6 2.4 2.3
9.6 5.4 3.6 2.2 2.3
" Dielectric constant values for 20-25",taken mainly from A. A. Maryott and E. R. Smith, "Table of Dielectric Constants of Pure Liquids," NBS Circular 514,1951. bI.Nakagawa and S. Mizushima, J. G e m . Phys., 21,2195 (1953). c S . Mizushima, T. Shimanouchi, I. Ichishima, and H. Kamiyama, Rev. Universelle Mines, 15, 447 (1959).
Table II. Solvent Effects on the C-CI Stretching Absorption Bands of 1,1,2,2-TetrachIoroethane
Ti Solvent
1 2 3 4 5 6 6' 7 8 9 10 11 12 13 14 15 16
Vapor n-Hexane Cyclohexane Pentene 1,4-Dioxane Carbon tetrachloride Benzene Benzene-d p-Xylene Mesitylene Tetrachloroethylene Triethylamine Carbon disulfide Diethyl ether Ethyl acetate Acetone Acetonitrile Nitromethane
T2
G1
vs ,c - CI (trans)' v, cm-' 103Av/v
V'16.C-CI
v, cm-'
Ratios of peak
G1
(gauche)" vl6.c-Cl (trans)' v'4.c- c1(gauche)" intensities 103Av/v v, cm-' 103Av/v v, cm-' 103Av/v AT?/A,, A T J A G ,
761" 758 757.5" 756.5 751
3.9 4.6 5.9 13.1
745" 739.5 739.5 739 735
7.4 7.4 8.1 13.4
722" 717 716 712.5
6.9 8.3 13.1
753.5 753.5 753 752.5
9.9 9.9 10.5 11.2
737 737 737 737 738
10.7 10.7 10.7 10.7 9.4
714.5 714.5 714.5 716
10.4 10.4 10.4 8.3
754.5
8.5
751.5
12.5
10.4 11.1 11.1 11.8 10.4
9.9
10.7 11.4 10.7 11.4 11.4 10.7
714.5 714 714 713.5 714.5
753.5
737 736.5 737 736.5 736.5 737
654" 651.5 651 650.5 645.5 650.5
3.8 4.6 5.4 13.0 5.4
647 647.5 647.5 650.5 645 649 646 645.5 644.5 645
10.7 9.9 9.9 5.4 13.8 7.6 12.2 13.0 14.5 13.8
0.81 0.77 0.76 0.55 0.55 0.53 0.62 0.74 0.74 0.61 0.42 0.37
1.07 0.95 0.61 0.66 0.67 0.75 0.99 0.91 0.66 0.57 0.41 0.40
0.34
" Reference 14. frequency shifts to those of n-propyl chloride as standard, rather than to those of the previously recommended16 cisdichloroethylene, because the interaction of the new reference compound with solvents is virtually completely electrostatic in nature, free of the hydrogen-bonding perturbations that affect cis-dichloroethylene. l 3
Experimental Section Measurements were carried out as described before.13 Transmittance values were reproducible to k 1 %. DCE (Fisher Certified Reagent) and TCE (Fisher Reagent Grade) were fractionally distilled from phosphorus pentoxide at atmospheric pressure. p-Xylene (Eastman White Label), mesitylene (Eastman Yellow Label), and pentene (Eastman Technical Grade) (16) H. E. Hallam and T. C. Ray, Trans. Faraday SOC.,58, 1299 (1962).
Oi, Coetzee
were dried over Linde 3A Molecular Sieves' and then fractionally distilled at atmospheric pressure. Hexadeuteriobenzene (Merck) was used without further purification. Other solvents were the same as before.13
Results and Discussion Tables I and I1 contain the observed C-C1 stretching frequencies of DCE and TCE, respectively, in wavenumber units, as well as the relative frequency shifts, 103Av/v,calculated on the basis of vapor-state frequencies, With TCE we made measurements on four bands, two for each rotational isomer. As reported before,14 two additional bands of the trans isomer are very weak, and two of the gauche form are very close together. In addition to the vc-cl values listed in our previous paper,13 some complementary data for n-propyl chloride and isobutyl
/ Solvent Eflects on 1,2- Dichloroethane
and I ,I ,2,2-Tetrachloroethane
2480 Table III. Solvent Effects on the C-Cl Stretching Frequencies of n-Propyl Chloride and Isobutyl Chloride f VC- CI
Solvent
v, cm-I
Vapor
7430 734 725' 725 726.5 727
3 Pentene 6 Benzene 6' Benzene-d 7 p-Xylene 8 Mesitylene
'
n-Propyl chloride vc;cI (gauche) (trans) 103Av/v v, cm103Av/v 12.1 24.2 24.2 22.2 21.5
660" 653.5
9.9
649 650 650.5
16.7 15.1 14.4
'
Isobutyl chloride vc-cI (gauche) vc-cI (trans) v, cm-l 103Av/v v, cm-I 103Av/v 745.9 734 726.5' 726.5 728.5 729.5
15.4 25.3 25.3 22.8 21.5
698b 690.5
10.7
685 686
18.6 17.2
"C. Komaki, I. Ichishima, K. Kuratani, T. Miyazawa, T. Shimanouchi, and S. Mizushima, BUN. Chem. Soc. Japan, 28, 330 (1955). Reference 13.
* Reference 7.
acceptor properties, such as triethylamine and diethyl ether, shows that the interaction of this solute with solvents is largely dielectric in nature. On the other hand, Figure 2 shows that several points fall well off the straight lines for each of the four bands of TCE. This behavior is similar to that reported by us for other chlorinated hydrocarbons containing active hydrogen atoms,13 and it indicates that TCE undergoes specific interactions with certain solvents. In the hydrogen-bond acceptor solvents triethylamine and diethyl ether, the main specific interT(trans) GI (gauche) Ge(gauche) action undoubtedly is hydrogen bonding, and it involves , , , , , , , , , I Q I both isomers of TCE. 15 20 25 15 20 25 15 20 25 Peak intensities were measured for both bands produced 103Av/v C-CI of 1.2-dichloroethane by each isomer, giving two corresponding sets of values for the ratio A,/& for DCE and also for TCE. Those values Figure 1. Relative frequency shifts of vc-cl bands of 1,2-dichlorothat could be determined with reasonable accuracy are ethane plotted against the corresponding shifts of n-propyl chloride; listed in Tables I and 11. Because of some overlap of numbers of solvents refer to Table I. bands, and because the absorptivity indices of the two isomers may differ somewhat, these numbers only give an approximate indication of the relative populations of the two isomers. Two main facts emerge. First, for both f 30 f 30 DCE and TCE in most solvents, the relative population of c E the gauche isomer increases with increasing dielectric 2 constant of the solvent, in agreement with previous conclusions for DCE4,'*' and TCEI4 in a limited number of = 20 : 20 r 0 solvents. Second, for both DCE and TCE in dioxane and c E in aromatic solvents, the relative population of the gauche e isomer is greater than would be predicted on the basis of .5 s the bulk dielectric constants of these solvents, in agreement 1 0 g 10 1',(trans) G,(gauche) A - 3 with previous results with DCE. Furthermore, this in1 crease in the gauche population is approximately the same a T2(trans) G&gouche) n for DCE and TCE, which is an important result, because it 0 0 L d L d provides an argument against the hypothesis discussed in 10 15 10 15 5 10 10 15 the introductory section that hydrogen bonding is responsi!03Av/vC-CI of 1,1,2,2-tetrachloroethane ble for the relative stabilization of the gauche isomer of DCEinaromatic solvents. There are also other arguments Figure 2. Relative frequency shifts of vc-cl bands of 1,1,2,2-tetraagainst the hydrogen-bonding hypothesis. First, the chloroethane plotted against the corresponding shifts of n-propyl C-Cl frequency shifts of n-propyl chloride and isobutyl chloride; numbers of solvents refer to Table 11. chloride are adequately represented by the KirkwoodBauer-Magat treatment in all solvents studied, except again in dioxane and benzeneI3 and other aromatic chloride in pentene and in aromatic solvents are given in solvents (Table 111). Clearly these two solutes also Table 111; these two compounds give essentially the same undergo a specific interaction with the solvents in quesresults in all solvents studied when used as references. tion, although their hydrogen-bonding capacity is negliIn Figures 1 and 2 the relative frequency shifts of the C-Cl stretching vibration bands of the rotational isomers gible. Second, the gauche isomer of both DCE and TCE of DCE and TCE, respectively, are plotted against the is stabilized to a much greater extent by acetonitrile and corresponding shifts of n-propyl chloride as reference nitromethane than by diethyl ether which is a much stronger hydrogen-bond acceptor but less polar than the (modified Bellamy-Hallam-Williams plots13). The reasonable linearity obtained in Figure l for each of the three first two solvents. That the interaction responsible for the stabilization of bands of DCE, even in solvents with strong hydrogen-bond
f
1
:
x
$
Journal of the American Chemical Society 1 91:lO May 7,1969
2481 the reference compound, n-propyl chloride, should be regarded as a measure of the “effective electrostatic 30 polarity’’ of the solvent. It should be noted that for the aromatic solvents studied, all of which have the same bulk 20 20 dielectric constant, this effective polarity increases in the 8 g order mesitylene < p-xylene < benzene (Table 111))which ’6 10 is also the order of increasing stabilization of the gauche isomer, as measured by the intensity ratio (Tables I and 11). These results are in harmony with previous observations 2 4 6 8 1 0 c that the dipole moments of DCE and TCE are larger in benzene than in p - ~ y l e n e . l * ” ~On the other hand, the order of hydrogen-bond acceptor power is the reverse of the above.” It is noteworthy that the linear relationship between relative frequency shifts and intensity ratios holds reasonably well even for dioxane, benzene, and p-xylene as solvents, in which n-propyl chloride or isobutyl chloride gives anomalously large frequency shifts. As before,’ we speculate that these solvents may be more polar locally than their macroscopic polarities would indicate. In 0.2 0.4 0.6 a8 1.0 0.2 0.4 0.6 0.8 1.0 i.2 1.4 agreement with the results of Mizushima, et u Z . , ~ the A T ,IAG, AT plAGp aliphatic double bond of pentene caused no anomaly. Ratio of peak intensity We conclude that the interaction of DCE with all solvents studied is mainly electrostatic in nature. In the Figure 3. Correlation between the ratio of peak intensities of case of TCE hydrogen bonding occurs significantly with vc- cI bands of rotational isomers of 1,2-dichloroethane or 1,1,2,2tetrachloroethane and the relative frequency shift of v ~ band - of ~ ~ proton-acceptor solvents, and it involves both isomers. n-propyl chloride. The relative populations of the two isomers appear to be influenced more markedly by electrostatic interactions than by hydrogen bonding. the gauche isomer of DCE and of TCE may be largely Acknowledgment. Financial support by the National electrostatic in nature is further suggested by the fact that a Science Foundation under Grant No. GP-6478X is reasonably linear relationship exists between the intensity gratefully acknowledged. ratios A,/A, of both DCE and TCE and the relative frequency shifts of n-propyl chloride. Such plots are (17) M. Tamres, J. Am. Chem. Soc., 74, 3375 (1952). shown in Figure 3. We believe that the frequency shifts of
t
30
0)
Bp
i
Oi, Coetzee Solvent Eflects on 1,2-Dichloroethane and 1,1,2,2-Tetrachloroethane
